Optical waveguide modulators are commonly used to modulate light generated by lasers and other light sources. In optical communications, different phase modulation schemes may be advantageously employed, which include Phase Shift Keying (PSK) methods such as Binary Phase Shift Keying (BPSK), Quadrature Phase Shift Keying (QPSK) and Differential Quadrature Phase Shift Keying (DQPSK). Return-to-Zero Phase-Shift Keying (RZ-PSK) is characterized by the phase modulation of a train of optical pulses, and may have beneficial properties in combating distortions seen in fiber optic cables at longer distances compared to a simpler Non Return-to-Zero (NRZ) PSK modulation, wherein light intensity remains unchanged. By using PSK-based communication schemes, the capacity and link performance can be enhanced in comparison with direct detection schemes utilizing On-Off amplitude keying.
In PSK modulation, data is transmitted by controlling the phase of an optical carrier, e.g. laser light, in accordance with the transmission data. For example, in QPSK modulation, the optical phase of the optical carrier is switched between four values “θ”, “θ+π/2”, “θ+π”, and “θ+3π/2”, where “θ” is an arbitrary phase, which are assigned respectively to two-bit symbols “00”, “10”, “11”, and “01”. A receiver device recovers the transmission data by detecting the phase of the received optical signal.
In DQPSK modulation, the transmitted data are differentially encoded, that is, they are represented by the difference in phase between successive symbol intervals. In this technique, in each successive symbol interval the modulator imparts one of four possible phase shifts (0, π/2, π, 3π/2) on the optical carrier, while the receiver measures the phase difference between two successive received symbols, so that the absolute phase of the optical carrier is not needed to decode the transmitted symbols.
Optical modulators for DQPSK and QPSK modulation are known in the art, and typically utilize waveguide structures formed in electro-optic materials such as LiNbO3 or compound semiconductors having suitably high electro-optic coefficients, for example GaAs or InP based. Conventionally, such modulators include two or more waveguide BPSK modulators, i.e. waveguide phase modulators that are driven by binary electrical signals and impart one of two phase shift values on light passing therethrough. The same optical structure can be used for either the QPSK or DQPSK modulation, with different pre-coding of electrical drive signals in each case.
A typical waveguide phase modulator includes a waveguide formed in or upon an electro-optic material disposed between a pair of electrodes extending alongside the waveguide adjacent thereto so as to induce an electrical filed in the waveguide. By applying a drive voltage across the electrodes, a change in the refractive index of the waveguide can be affected, thereby changing the optical phase acquired by guided light at the output of the phase modulator.
One common type of (D)QPSK modulators utilize a Mach-Zehnder (MZ) waveguide structure, wherein output ports of an optical splitter are connect with input ports of an optical combiner by two waveguide arms. Mach-Zehnder electro-optic modulators (MZMs) are widely used as optical intensity modulators and have an optical transmission versus drive voltage characteristic which is cyclic and is generally raised cosine in nature. The half period of the MZM's characteristic, which is measured in terms of a drive voltage, is defined as Vπ. In order to operate as a QPSK or DQPSK modulator, each MZ arm includes a phase modulator driven by a data signal that may impart either a 0 or π phase shift upon light propagating in the respective arm, with one of the arms including an additional π/2 phase shifter.
For example, US Patent Publication 2004/0081470 to Griffin discloses such an optical QPSK modulator wherein the phase modulators in the waveguide arms are in turn MZMs that are biased for minimum optical transmission in the absence of a drive voltage and are driven with respective drive voltages VI(t), VQ(t)=+/−Vπ to give abrupt phase shifting with a minimum of amplitude modulation. Such an MZM based phase modulator produces light wherein the optical phase abruptly switches by π radian, crossing a zero intensity. One disadvantage of this MZM-based phase modulator is the appearance of a third harmonic of the modulation frequency in the optical spectrum of the output light. A further disadvantage of this scheme, is that the MZM must be driven to 2·Vπ drive voltage in order to produce the 0 to π phase shift.
It is also known to sequentially connect two or more binary phase modulators to provide multi-level phase modulation of light. US Patent Application No 2004/0141222, in the names of T. Miyazaki and K. Kikuchi, discloses an m-ary PSK modulator that produces multi-level phase modulation by utilizing a plurality of binary phase modulators disposed in series, wherein n-th phase modulator produces a phase shift of either 0 degrees of 2nφ degrees; here, φ is a predetermined phase level. For example, by using two phase modulators connected in series, either DQPSK or QDPSK modulation can be realized.
To provide a high-quality DQPSK or QDPSK signal and ensure error-free reception of transmitted signal, it is important that the phase shifts imparted by the phase modulators are equal or very close to design values. If uncontrolled, the phase shifts imparted by the phase amplifiers may vary with time, for example due to device aging or changes in environmental conditions such as temperature, which may cause changes in material properties of the waveguide or in characteristics of driving circuitry. Therefore, there is a need to monitor the phased shifts imparted by the device to ensure its correct operation. One problem with using sequentially connected phase modulators to modulate the optical phase of light is that the optical phase is considerably more difficult to monitor than the light's intensity. A conventional photodetector capable of detecting light intensity will not respond to the phase shift portion of the optical signal.
An object of the present invention is therefore to provide a waveguide optical device that includes a waveguide phase modulator and integrated means for monitoring the optical phase shifts imparted by the waveguide phase modulator upon light propagating therethrough.